SURVEY OF CELL-LIFE. 169
so large a quantity present, that the cells contained in it do not come into contact, as is the case in most cartilages. The chemical and physical properties of the cytoblastema are not the same in all parts. In cartilages it is very consistent, and ranks among the most solid parts of the body; in areolar tissue it is gelatinous; in blood quite fluid. These physical distinctions imply also a chemical difference. The cytoblastema of cartilage becomes converted by boiling into gelatine, which is not the case with the blood; and the mucus in which the mucus-cells are formed differs from the cytoblastema of the cells of blood and cartilage. The cytoblastema, external to the existing cells, appears to be subject to the same changes as the cell-contents; in general it is a homogeneous substance; yet it may become minutely granulous as the result of a chemical transformation, for instance, in areolar tissue and the cells of the shaft of the feather, &c. As a general rule, it diminishes in quantity, relatively with the development of the cells, though it seems that in cartilages there may be even a relative increase of the cytoblastema proportionate to the growth of the tissue. The physiological relation which the cytoblastema holds to the cells may be twofold : first, it must contain the material for the nutrition of the cells; secondly, it must contain at least a part of what remains of this nutritive material after the cells have withdrawn from it what they required for their growth. In animals, the cyto- blastema receives the fresh nutritive material from the blood- vessels ; in plants it passes chiefly through the elongated cells and vascular fasciculi; there are, however, many plants which consist of simple cells, so that there must also be a transmission of nutrient fluid through the simple cells; blood-vessels and vascular fasciculi are, however, merely modifications of cells.
Laws of the generation of new cells in the cytoblastema.— In every tissue, composed of a definite kind of cells, new cells of the same kind are formed at those parts only where the fresh nutrient material immediately penetrates the tissue. On this depends the distinction between organized or vascular, and unorganized or non-vascular tissues. In the former, the nutritive fluid, the liquor sanguinis, permeates by means of the vessels the whole tissue, and therefore new cells origi-
